Balancing of capacitor voltages is an issue which can naturally arise in multilevel topologies, or whenever the DC-link voltage is split in a topology for a special purpose, for instance, to allow connection to a neutral grid point. Solutions for this issue have been largely reported in the literature [1]-[2].
In document [1], the authors propose use of an energy function which is minimized in real time by evaluating redundant control vectors of a multi-level converter. A similar idea is followed in document [2] where the authors explore two strategies also based on the use of redundancies and minimizing a cost function in terms of the voltage imbalance. In document [3], capacitor voltages are balanced using redundant states for five-level converters. In document [4], a modulation strategy for guaranteeing balancing of DC-link capacitor voltages is presented. The proposed strategy in document [4] is based on a modified discontinuous PWM.
Another issue in three-phase inverters involves the generation of common-mode voltage (CMV). This issue has attracted attention recently, since it has become more evident, for example, in transformerless applications, where no galvanic isolation is available as described in document [5]. CMV is an issue which can be associated with the inverter topology and the modulation algorithm used. CMV can, for example, be induced by an inverter itself, and can then be propagated to the equipment connected to the inverter, causing severe adverse effects.
The CMV can manifest itself as a zero-sequence voltage fluctuation with respect to ground. The zero-sequence voltage fluctuation can, depending on the application, cause indirect grid current distortion, additional losses and safety issues, among other things as described in document [6]. For instance, CMV in electric drives can create bearing currents which can cause physical damage to electrical machines. CMV can also cause considerable leakage current to flow through the parasitic capacitances of photovoltaic (PV) panels supplying an inverter.
A known solution to the CMV problem involves splitting the DC-link of the inverter into two halves and connecting the mid-point of the DC-link to the neutral point of the grid as described in document [7]. After this modification, each leg in the inverter bridge is controlled independently as if they were three independent single-phase systems. However, this modification reduces utilization of the DC voltage, e.g., it limits the modulation index. Moreover, an additional balance strategy can be specified to guarantee that both halves in the DC-link maintain the same voltage.
In multi-level inverters, there can also be a possibility to eliminate the CMV by avoiding certain switching control vectors which produce CMV. In document [8], it is shown that inverters with an odd number of levels can avoid generating common-mode voltage by switching among certain available states.
However, when switching states are restricted, it is no longer possible to guarantee the balance of capacitor voltages. DC-link voltage balancing (or neutral point balancing) and CMV cancelation cannot be achieved concurrently without hardware modifications as described in document [11]. Thus, some authors have proposed inserting a fourth leg to handle the DC-link voltage balancing issue as described in documents [9]-[11]. The added fourth leg can, however, increase the complexity and decrease the cost-effectiveness of the inverter.
Another approach is disclosed in document [12], where a filter is proposed for a three-phase adjustable-speed motor drive. The filter can be disposed in a three-phase LRC network at the output of a two-level inverter, where a filter star point is electrically connected to a DC-link midpoint, thus capacitively forming an artificial mains neutral star point. A similar idea is used in a rectifier system of document [13]. The filter can passively reduce both the differential and the CMV without an added fourth leg. On the other hand, resistors in the filter introduce additional losses to the system, thus, reducing the efficiency of the system.